From the membrane fraction of the Gram-positive bacter-ium Carboxydothermus hydrogenoformans,anenzyme complex catalyzing the conversionofCOtoCO2andH2was purified. The enzyme complex showed maximal CO-oxidi-zing:H2 -evolving enzyme activity with 5% CO in the head-space (450 U per mg protein). Higher CO concentrations inhibited the hydrogenase present in the enzyme complex. For maximal activity, the enzyme complex had to be activated by either CO or strong reductants.